Pulse oximetry is a process for measuring arterial oxygen ration. It can be employed in principle in all pulsatile (systolic-diastolic blood pressure) perfused tissues of all species.
Description of the Related Art
Pulse oximetry has been employed clinically with great success for many years. It is no longer possible to imagine operating theaters, intensive care stations, emergency rooms, ambulances, obstetrics wards, etc. without it. Its success rests on its simple operation, the reliable indication and the uncomplicated interpretation of the indicated values.
Pulse oximetry has, until this day, not been available for use as an indicator during delivery. This is remarkable, because oxygen saturation is an essential piece of information for the birth helper,
first, in order to make possible the earliest possible diagnosis of problems relating to oxygen condition, and PA1 second, in order to evaluate more precisely the classical cesarean section indicators (16% Germany, 20% Europe, 25-33% USA). PA1 1. Here young healthy subjects inhale a mixture of oxygen and nitrogen (and carbon dioxide), of which the oxygen partial pressure is stepwise lowered in defined steps from normal values to hypoxic values. After an equilibration is achieved between the oxygen partial pressure in the gas mixture and in the blood, blood samples are taken, from which the oxygen saturation in the arterial blood is determined and at the same time, that is, at the moment the blood is withdrawn, the measured value Omega (see below) is measured with the pulse oximeter to be calibrated. This calibration is thus developed by obtaining as many solid measured values as possible, for which both the oxygen saturation as well as the Omega value is known. PA1 1. light leaving the emitter is radiated in all directions, PA1 2. the dye, blood, or blood perfused tissue is present in all spatial directions, and so absorbs as well as scatters, PA1 3. there is not a geometric border such as that produced by a cuvette, but rather the boundaries of the three-dimensionally transilluminated tissue are determined by intensity attenuation, that is, with corresponding light pathlengths, over which the light is attenuated by scattering and absorption, the light intensity is so reduced, that finally it's contribution to the total light signal is irrelevant (one could call this an "intensity limited cuvette" , see FIG. 1). PA1 1. the scattering level of tissue is achieved, which is significantly higher than that of blood alone, and PA1 2. a substantial independence from the scattering effect of blood is achieved, which can strongly fluctuate with the hemoglobin content, or more precisely, the cell count. PA1 at least one subassembly for adjusting the concentration of a substance to be detected in a bodily fluid; PA1 at least one transport subassembly, with which the bodily fluid adjusted to a specific concentration can be brought to at least one measurement cell; PA1 at least one emitter and at Least one receiver, which are provided in the measurement cell, so that their mean effective light path to each other can be actively and dynamically altered in a defined manner, without requiring the bodily fluid to transmit forces for altering the mean effective light path between light emitter and light receiver; and PA1 a device which measures the intensity of light, which has passed through the bodily fluid, detected in at least one spectral window, in a form providing at least one suitable parameter and producing an association between the concentration on the one hand and the parameter on the other hand. PA1 at least one emitter and at least one receiver, which are provided in the measuring cell, wherein in the optical space between emitter and receiver an optical discontinuity of variable thickness is provided. PA1 in-vitro setting or adjustment of a particular oxygen saturation value in blood; PA1 transportation of the blood adjusted to a particular oxygen saturation to at least one measuring cell; PA1 measuring a light intensity in at least one spectral window for determination of at least one suitable parameter in the blood, which is contained in the measuring cell, via at least one emitter and at least one receiver, PA1 a container with at least one supply and at least one removal opening as well as at least one emitter and at least one receiver for light waves, PA1 an effective absorption length through the bodily fluid, which is actively and in a dynamic manner altered in a defined way without using the bodily fluid to transmit forces, wherein the bodily fluid occupies the optical space between the emitter and receiver. PA1 existing brain damage PA1 anencephally PA1 decoupling tissue from a healthy brain with it's own circulation system, for example, via a heart-lung machine perfused extremity (arm or leg) PA1 It simulates an arterial/arteriole-pulsation in that between light emitter and light receiver a change of distance can be produced, of which the intensity (amplitude) and frequency can be adjusted over a great range. PA1 It simulates the arterial character in that pulsating, arterial blood, that is, blood immediately after the gas exchange in the lung, courses through the tissue model. As a result, each oxygen saturation condition in the blood can be simulated. PA1 It makes possible the application of the fetal transmission-pulse oximetry-sensor (and other reflection sensor and transmission-pulse oximetry-sensors) so that the sensor completely and exclusively evaluates the arterial blood in the tissue model. PA1 It enhances the reproducibility and precision (oxygen saturation, heart frequency, pulse form, pulse harmonics), than is possible with natural tissues. Of particular interest is the sinusoidal-shaped optical plethysmography and the resulting purity of the frequency spectrum. PA1 It makes possible determinations independently of biological characteristics of natural tissue such as edema, hematoma, injuries, and from physiological values, such as blood pressure, heart frequency, blood flow. Each of these values can be adjusted independently and maintained constant. PA1 It avoids the necessity to set the oxygen saturation by means of the lung (brain damaging), that is, a steady state through inhalation of a defined adjusted mixture (nitrogen, oxygen, carbon dioxide). PA1 It makes possible determinations independent of movement artifacts, from outside light sources, temperature variations and restricted perfusion (shock). PA1 It provides the necessary transparency and reproducibility during the tests. PA1 It makes possible the imitation/simulation of any living species in that the species-specific blood (hemoglobin) is being utilized. PA1 final testing for a pulse oximiter manufacturer, PA1 monitoring by regulatory authorities, and PA1 precise function tests for recurring tests by the end user. PA1 separation of the light path, PA1 reception of the light amplitude, PA1 recognition of which LED is irradiating, PA1 multipliers for each individual LED amplitude, PA1 multipliers selected so that base absorption, heart beat, perfusion, finger thickness, movement artifacts, smoking, disturbances and other artifacts can be simulated, and PA1 generation of a true light signal with one or more LEDs, which is proportional to the initial light and the respective multipliers. PA1 base absorption PA1 heart beat PA1 perfusion PA1 finger thickness PA1 motion artifacts PA1 smoking PA1 disturbances PA1 other characteristics PA1 Fully valid values are obtained with no constraint as a result of intolerance of low oxygen saturation condition or, as the case may be, through movement artifacts. PA1 In a further embodiment according to the invention, the variance of the absorption is achieved by introduction of a transparent body, for example a glass wedge or a glass step, which displaces blood. In this manner a blood layer thickness change is produced. A servomotor with an alternating direction of rotation and a coil spool for transformation of the rotation movement into a translational movement moves the optical wedge between light emitter and light receiver. This calibration principle can be used with fetal pulse oximetry sensors in which both light emitter as well as light receiver are placed within the tissue, that is, are introduced via a spiral needle. These so-called inside-inside sensors can be constructed in the following manner: in the part of the spiral, which comes to rest within the tissue in the operational state, two oppositely lying opposing windows are formed in the needle. In one window the two LEDs are provided, and in the other a small photodiode is placed. PA1 The entire system is an artificial circulatory system, that is, the components are positioned analogously to their positions in the circulatory system in the organisms in a circular manner. The blood is thus pumped in a circulatory system. PA1 fractional oxygen saturation, and PA1 functional oxygen saturation. PA1 I.sub.0 =intensity of the entering light PA1 c=concentration of the dye stuff (independent of the wavelength) PA1 d=layer thickness of the absorbing dye stuff (dependent upon wavelength by scattering) PA1 e=specific absorption coefficient (valid for a specific wavelength only) PA1 1. Actual human tissue (subjects, patients) of an intact, organism capable of living , normally perfused, that is, by a viable heart, PA1 2. Actual human tissue of an organism not capable of living in the end (brain dead, anencephaly) normally perfused, that is, by means of an intact heart, eventually also artificially perfused (heart-lung machine). Ethics? PA1 3. Animal tissue, PA1 4. Artificial tissue, that is, tissue model for pulse oximetry, that at least as a minimal condition possesses the essential characteristics of arterioles, since arterioles are the anatomic structure of tissue pulsatility, and therewith are the cause for the optical plethysmography. The artificial tissue model must also thus at least possess the following characteristics or components: PA1 I.sub.0 =incident light intensity PA1 I.sub.out =emerging light intensity PA1 c=concentration of the absorbing medium PA1 e=extinction co-efficient of the absorbing medium PA1 d=layer thickness PA1 .delta.=supplemental layer thickness EQU I.sub.out max =I.sub.o *e.sup.-.epsilon.*c*d EQU I.sub.out min =I.sub.o *e.sup.-.epsilon.*c*(d+.delta.) PA1 optically homogeneous medium PA1 no optical scattering PA1 monochromatic light PA1 only the medium to be measured pulsates, or the other pulsating parts can be ignored. PA1 The scattering is increased or decreased depending upon whether the scattering effect of the scattering/absorption body is greater or less than the displaced bodily fluid. PA1 The layer thickness of the bodily fluid is decreased. Since, however, preferably the absorption of the scattering/absorption body is selected to lower than the absorption of the bodily fluid, it follows that
It would thus be expected that numerous research groups would be working in order to make this process available for use during delivery. An author of the present patent application has been awarded a pioneer patent for transmission pulse oximetry on unborn fetuses during birth.
During the development of pulse oximetry for assistance in delivery, a particular difficulty has become apparent: calibration. By this, we refer to the achievement of a correlation between the fetal oxygen saturation indicated by the pulse oximeter and the actual arterial oxygen saturation found in the blood of the fetus. Calibration becomes particularly difficult in the fetal area because the fetus already has, or can have, a physiologically very low oxygen saturation. One must thus be able to calibrate, among other things, for very low arterial oxygen saturation levels, which are not reconcilable with those of normal living organisms (be they human or animal), at least not in all situations, in which the brain is included in the low arterial oxygen saturation levels. This does not apply when, for example, a body part, such as an arm, is temporarily isolated for calibration/validation, that is, has a circulation rate differing from the rest of the body which is in correspondence with that of the brain. The fetus is equipped, for this physiological oxygen poor condition, with a special hemoglobin variant, the fetal hemoglobin (HbF). It exhibits a left-biased oxygen binding curve, and thus has a particularly high affinity for oxygen and makes possible therewith even under physiologically non-optimal conditions (prolonged phases of oxygen depletion e.g., labor pains, expulsion period!, difficult diffusion placenta!, mixture of arterial with venous blood physiological shunt!), bonding with oxygen which makes oxygen ultimately available to the fetal tissue.
Because all pulse oximeters available on the market must be calibrated by the manufacturer, a number of methods have been developed and described in the literature. The calibration over a great oxygen saturation range, as is necessary for the fetal situation, provides, however, a particular requirement or demand on the manufacturer of the respective devices, which cannot be overcome with known methods.
The state of the art describes various calibration methods:
1) Calibration on voluntary subjects:
A table or curve is then input into the device, that is, the software of the device, which makes it possible to determine at any measured Omega the corresponding oxygen saturation (Severinghaus/San Francisco: Pulse Oximetry, Springer Verlag, 1986).
2) Another method comprises photometricaly measuring the pulsatile arterial blood in an artificial construct instead of in a living tissue. For example, an artificial finger is made of a red casting resin. A hole is bored into the transplant material into which a shaft is introduced, onto the end of which a slit is sawed. In this slit a red filter disc is wedged. When this test finger is introduced into a pulse oximetry finger sensor and the shaft is rotated, transmission fluctuations are produced as a result of the angular dependent change of the color-dye layer thickness, which can imitate a tissue. As described, depending on the colored dye concentration in the filter disc, oxygen saturation values from 50 to 100% can be imitated. A. J. Munley, A. Shaw, M. J. Sik in The Lancet, May 13, 1989. In comparison to an actual finger, no changes occur in the intermediate effective light path between the light transmitter and light receiver, since the finger remains unmoved. This system is very artificial. Often there remains the question of how the test finger itself is to be calibrated.
Because the physiological and/or pathologic oxygen saturation in unborn fetuses can be less than 10%, and such low oxygen-saturation values are not reconcilable with life after birth, there is a requirement for a completely new calibration concept. An essential fundamental principle of this calibration concept must be to find a tissue or, as the case may be, a tissue model, in which the arterial oxygen saturation over the entire range from 100% down to (close to) 0% can be imitated, without any concern that the lack of oxygen saturation affects the validity of the model.